The present invention generally relates to a flat antenna apparatus, and more particularly to a flat antenna apparatus which receives a radio frequency by using a flat antenna element generally used for a Global Positioning System (GPS).
The GPS used for, for example, a car navigation system utilizes a radio frequency transmitted by a plurality of GPS satellites to the ground so as to locate a current position (latitude, longitude and altitude) of a moving object. The GPS generally comprises an antenna unit, a receiving unit, a signal processing unit and a display unit.
A radio frequency in a Giga-Hertz band transmitted by the satellites is received by the antenna unit. The received radio frequency is amplified by a low noise amplifier, and thereafter supplied to the receiving unit as a received signal. The receiving unit demodulates the signal supplied by the antenna unit, and supplies it to the signal processing unit. The signal processing unit performs a signal processing by using the demodulated signal received from the receiving unit. The display unit displays on a map a result of a position determination in accordance with the signal processing.
FIG. 1 is a side view of a conventional flat antenna apparatus 10.
The flat antenna apparatus 10 comprises a flat antenna element 11, a circuit board 14 and a mounting board 15. The flat antenna element 11 is formed by attaching a patch antenna 13 made of a gold plated copper plate to a bottom surface of a board 12 made from a fluorocarbon resin. The flat antenna element 11 is fixed on a bottom surface of the mounting board 15 by an adhesive tape.
A low-noise amplifier is mounted on the circuit board 14 which low-noise amplifier is connected to the patch antenna 13. The low-noise amplifier amplifies a received radio frequency supplied by the patch antenna 13. The circuit board 14 is placed on top of the mounting board 15 which is made of a metal, and fixed to a bottom surface thereof by an adhesive. FIG. 2 is a perspective view showing a positional relationship between the antenna 11, the mounting board 15 and the circuit board 14. It should be noted that the drawing of FIG. 2 is illustrated upside down relative to FIG. 1
The mounting board 15 is grounded so as to shield the flat antenna element 11 and circuit board 14. Additionally, a coaxial cable 18 is provided for connecting the low-noise amplifier mounted on the circuit board 14 to an external receiving unit.
In the above-mentioned flat antenna apparatus, the mounting board 15 is surrounded by a metal case 16, and a cover 17 is attached thereon so as to cover the circuit board 14. Accordingly, a shielding effect is obtained by providing the mounting board 15 made of a metal while the good mountability of the metal case 16 is maintained.
In the flat antenna apparatus 10, since the circuit board 14 and the mounting board 15 are placed on top of each other, a gap is formed therebetween when the circuit board 14 or the mounting board 15 is warped. In this condition, the low-noise amplifier which has a high gain may have a drift capacity, and thus there is a problem in that an abnormal oscillation is generated in the low-noise amplifier.
In the flat antenna apparatus 10, the case 16 is electrically connected to the circuit board 14. That is, the case 16 is connected to a ground pattern 14a formed on the circuit board 14 so as to eliminate an external noise via the case 16. This electrical connection is performed by means of a soldering at a predetermined position as shown in FIG. 3. The soldering operation is performed while abutting a solder iron against an inner wall of the case 16. After the soldering operation is finished, the cover 17 is attached to the case 16.
In order to perform the above-mentioned soldering operation, the case 16 and the cover 17 are formed as separate parts. Accordingly, the number of parts and the number of assembling processes of the flat antenna apparatus 10 are increased, and thus there is a problem in that a manufacturing cost of the flat antenna apparatus 10 is increased. Additionally, since the soldering operation is inconvenient, a reliable soldering may not be performed.
In FIG. 1, the coaxial cable 18 is fixed on an extending portion 14b of the circuit board 14 which extending portion extends outwardly from the circuit board 14. More specifically, a mounting portion 19 is provided in an area from which the coaxial cable 18 extends so that the mounting portion 19 pinches a shield conductor 18b of the coaxial cable 18. A core wire 18c of the coaxial cable 18 is soldered to the circuit board 14. The coaxial cable 18 is fixed to the extending portion 14b by using a fastener 20 which is a part separate from the circuit board 14 and the mounting board 15. The fastener 20 is fixed on the extending portion 14b of the circuit board 14 by means of screws 20a.
As mentioned above, in the conventional flat antenna apparatus 10, the coaxial cable 18 is fixed to the extending portion 14a by the fastener 20 and the fastener 20 is fixed on the extending 14a of the circuit board 14 by means of the screws 20a. Accordingly, the number of parts and the number of assembling processes are increased, and thus there is a problem in that a manufacturing cost of the flat antenna apparatus 10 is increased.